A mass spectrometer of the type considered herein generally consists of an ion source of any chosen design, for example, an ionization chamber for chemical ionization, as described in the German Offenlegungsschrift of Patent Application P No. 27 37 852.5; a mass filter, particularly a quadrupole mass filter of the type described in the German Patent No. 444,900, for example, as produced according to the German Offenlegungsschrift of Patent Application P No. 27 37 903.9 and its first patent of addition; and an ion detector.
It is known to arrange in axial alignment the ion sources, mass filters and detectors in the recited order, this arrangement generally being called an "in-axis" arrangement. This arrangement has the disadvantage that, due to the just described direct "visual contact" of source and ion detector, all neutral particles which, coming from the source, travel through the mass filter without being influenced and reach the detector. Depending on the type of the detector, this feature can generate an undesired background current in the detector. Such neutral particles could be, in particular, excited or metastable, neutral molecules and photons in the ultraviolet or soft X-ray range. A secondary-electron multiplier is usually used as detector.
To reduce this unwanted background current which occurs due to the direct "visual contact" between source and detector, the detector was laterally staggered relative to the axis of the mass filter. This arrangement is called an "off-axis" arrangement. Ionic current measurements which reach the indicating limit of a mass spectrometer, in particular, make such an "off-axis" arrangement of the detector necessary in order to improve the signal-background ratio of the detector.
For this purpose, it is necessary to ion-optically couple the mass filter, particularly a quadrupole mass filter, and "off-axis" ion detector. This is primarily done in a known manner by electrostatic deflection fields which are generated by suitably shaped sheet metal electrodes. Moreover, arrangements are known which solely utilize the attractive action (to positive ions) of the high negative potential of the first dynode of a secondary-electron multiplier. This potential typically lies in the range from 2000 to 3000 volts.
However, in such deflection arrangements, there is the disadvantageous effect that all multipole mass filters which have heretofore become known and are used in mass spectrometers have the property that the ions to be examined, upon emerging from the mass filter, have an energy distribution and an angular distribution which depend on the ion mass and the phase of the multipole-high frequency fields. As a result, all "off-axis" deflection arrangements which correspond to the present state of the art and, which are merely based on electrostatic repulsive or attractive fields, lead, through the mass-dependent energy and angular distribution of the ions, to a more or less large mass dependency of the ion deflection. Particularly disadvantageous is the insufficient degree of efficiency in "off-axis" arrangements of the design which is used today.